The present invention relates to a method of admixing a finely divided dye stuff to a stream of synthetic granulate passing through a fall shaft into an extruder, particularly a synthetic fiber extruder. The invention relates also to a device for carrying out this method.
From the German publication DE-OS No. 16 29 721 devices for dyeing a synthetic granulate fed into screw-type extruders are known. In these devices in which this invention has originated, a charging funnel is seated at the rear end of the extruder and communicates with an upright filling shaft for the synthetic granulate. A horizontal feeding shaft equipped with machine or conveying means for the dye stuff opens laterally into the filling shaft. In an exemplary embodiment of this prior art device, the dye stuff is in the form of a cylinder which is pressed by a spring against an abrasive disc; the separated particles pass through a downwardly sloping short channel into the filling shaft. In another embodiment the filling pipe for the dye stuff is provided with a screw conveyor and the dye stuff is supplied from a container or scraped off from a cylinder-shaped dye stuff body and again fed through a downwardly inclined channel into the filling shaft. In these prior art embodiments only a coarse metering or dosing of the dye stuff is possible. However, such a coarse dosing does not meet requirements in the production of high quality synthetic fibers.
German publication DE-OS No. 21 62 308 is concerned with a method for producing multicolor fibers of irregularly changing colors. In this known method, different dye stuffs in the form of pellets are supplied at predetermined time intervals in a statistical sequence through a charging pipe which is separated from the so-called supply pocket for the synthetic granulate, into the upstream end of the screw extruder so as to fall into an empty helical section of the screw extruder. The rotating screw of the extruder conveys the charge of pellets into the supply pocket where they are mixed with the synthetic granules and subsequently the mixture is conveyed into the melt to be extruded. This kind of feeding of the dye stuff is suitable for special applications. However, to achieve a uniformly dyed synthetic fibers used for spinning, the requisite uniformity of the mixture is not obtained, especially in the case when the dye stuff is for example in the form of granules or powders which tend to form lumps or nodules, or plugs or bridges in the screw extruder or in the charging pipe. Moreover, when the dye is changed the cleaning of the rear end of the screw extruder is extremely difficult.
Since the prior art devices for admixing a dye stuff into a synthetic granulate do not enable a uniform and exact admixing of the dye stuff, it is conventional in spinning plants to equip the inlets to the extruder with an additional mixer into which metered quantities or doses of the synthetic granulate and of the dye stuff are supplied (DE-OS No. 24 16 248, DE-OS No. 27 37 868; DE-OS No. 31 46 667). However, even these known solutions, in spite of increased expenditures, do not always yield satisfactory results. For example, due to differences in the size and density of the granules, and due to electrostatic charges there may occur dissociation or separation processes in the mixer leading to an irregular coloration of the melt.